Change "typedef" to "class" in core library and related tests.

tests/language/mixin_type_parameters_mixin_test.dart

Index: tests/language/mixin_type_parameters_mixin_test.dart
diff --git a/tests/language/mixin_type_parameters_mixin_test.dart b/tests/language/mixin_type_parameters_mixin_test.dart
index 1e8df1d690dfad05c9330b95db81a203376803a4..d5ddd6dc8e189eadb63dce48ebe856ecc534eb68 100644
--- a/tests/language/mixin_type_parameters_mixin_test.dart
+++ b/tests/language/mixin_type_parameters_mixin_test.dart
@@ -40,10 +40,10 @@ class M<T> {
 class S {
 }
 
-typedef C0<T> = S with M;
-typedef C1<T> = S with M<T>;
-typedef C2<T> = S with M<int>;
-typedef C3 = S with M<String>;
+class C0<T> = S with M;
+class C1<T> = S with M<T>;
+class C2<T> = S with M<int>;
+class C3 = S with M<String>;
 
 main() {
   var c0 = new C0();